Physical Defenses Against Herbivory in Gesneriaceae Meagan Turner Department of Biology, Washington State University ABSTRACT I studied physical plant defenses in seven species within the family Gesneriaceae: Alloplectus tetragonus, Besleria princeps, Besleria solanoides, Besleria triflora, Columnea glabra, Columnea microcalyx, and Monopyle maxonii. The species display varying levels of pubescence and leaf toughness. The purpose here is to determine if these physical traits deter herbivores and decreas e the amount of herbivory. Data were collected on plants growing along the Senderos Principal and DivisiÃ³n behind the EstaciÃ³n BiolÃ³gica, and the Sendero Nuboso in the Monteverde Cloud Forest Reserve, Costa Rica, from April 16 to May 8, 2006. Twenty indivi duals from each species were sampled and the percent herbivory and the leaf toughness were determined for each leaf. Leaf surface categories were distinguished based on the density of pubescence; glabrous one species, medium pubescence four species, an d high pubescence two species. Results showed that as pubescence increased, percent herbivory increased, and as pubescence increased, leaf toughness also increased. Results also showed a significant difference across and between species in regards to per cent herbivory and leaf toughness, with C. glabra being the toughest and B. princeps the most pubescent. There was no significant relationship between leaf toughness and herbivory. The study demonstrated that pubescence is not an effective defense against herbivory for the family Gesneriaceae. Furthermore, toughness is not related to percent herbivory, thus suggesting there may be other factors involved such as palatability or secondary compounds that deter herbivores from the less tough leaves. Lastly, dif ferences among species in these defensive traits further illustrate the trade offs that plants perform in order to maximize their efficiency. RESUMEN EstudiÃ© las defensas fÃsicas de las plantas de siete especies en la familia Gesneriaceae Alloplectus tetragonus, Besleria princeps, Besleria solandoides, Besleria triflora, Columnea glabra, Columnea microcalyx, Monopyle maxonii. Las especies presentaron niveles diferentes de pubescencia y dureza de la hoja. El propÃ³sito fue determinar si estas caracterÃs ticas fÃsicas pueden disuadir a los herbÃvoros y reducir la cantidad de herbivorÃa. Se recolectaron datos de plantas creciendo por los senderos Principal y DivisiÃ³n EstaciÃ³n BiolÃ³gica y el sendero Nuboso Reserva del Bosque Nuboso de Monteverde del 16 de abril al 8 de mayo del 2006. Veinte individuos de cada especie fueron examinados y la cantidad de herbivorÃa y la dureza de la hoja fueron determinadas para cada hoja. Se distinguieron diferentes categorÃas des superficies de las hojas: glabra una espe cie, de pubescencia mediana cuatro especies y muy pubescente dos especies. Los resultados mostraron que cuando la pubescencia aumentÃ³, la cantidad de herbivorÃa tambiÃ©n aumentÃ³, y que cuando la pubescencia aumentÃ³ la dureza de la hoja aumentÃ³ con ella . Las resultados mostraron una diferencia significativa entre y dentro de especies de acuerdo a la cantidad de herbivorÃa y la dureza de la hoja, con C. glabra presentando la mayor dureza y B. princeps la mÃ¡s pubescente. No hubo diferencias significativas entre la dureza de la hoja y la herbivorÃa. El estudio demostrÃ³ que la pubescencia no es una defensa efectiva en contra de la herbivorÃa para la familia Gesneriaceae. AdemÃ¡s, la dureza de la hoja no estÃ¡ relacionada con la cantidad de herbivorÃa, sugiriend o que quizÃ¡s hay otros factores como sabor o compuestos secundarios que reducen las visitas de los herbÃvoros a las hojas mÃ¡s suaves. Finalmente, las diferencias de estas caracterÃsticas defensivas en diferentes especies ilustran los compromisos que las pl antas deben ha cer para mejorar su eficiencia.
INTRODUCTION Both herbivory and antiherbivore efforts are costly to plant communities. Thus, populations must find a balance between allocating energy for functions such as growth and reproduction, and defe nse against herbivory Coley et al. 1983. Herbivory is the consumption and/or destruction of vegetative plant tissue by organisms such as snails, insects, mammals or other vertebrates. Herbivores exert a strong selective influence on plants by feeding on vegetative organs, thus removing biomass that might be allocated to growth or reproduction Coley et al. 1985. By feeding on the leaves, the herbivore directly reduces fitness by decreasing the leaf area and thus the photosynthetic ability of the plant. Indirectly, the herbivore could be affecting the reproductive capacity because resources may be diverted from flower and fruit production into defense and new leaf production Osborne 2000. Therefore, it may be beneficial for plants to utilize antiherbivo re mechanisms to improve their fitness. Antiherbivore defenses come in a myriad of forms. Nutritional inadequacy, or providing low essential nutrients to herbivores, physical barriers, and toxins are several mechanisms to deter herbivores Fitter and Ha y 1987. Physical barriers can present problems for organisms attempting to penetrate plant tissues. Thorns and trichomes are often effective physical barriers that come in a variety of shapes, sizes and densities. Densely packed hairs may physically bar i nsects from the underlying vulnerable tissues Herrera and Pellmyr 2002. Lignin provides structure and strength to leaves which makes it difficult for herbivores to chew the desired vegetative organs. Thus varying leaf toughness also provides protection a gainst herbivory. The family Gesneriaceae is found primarily in moist environments from tropical rain forests of the lowlands to temperate forests of the mountains and consist of both terrestrial and epiphytic growth forms Denham 2004. Gesneriaceae bo asts species that employ different types and intensities of physical defenses against herbivory. The most striking is the familyÂ€s variable pubescence. Leaves range from glabrous smooth to densely pubescent. Species also display varying levels of leaf to ughness. Some species possess succulent leaves and others characteristically thin leaves Gentry 1993. The diversity of these physical defenses raises questions regarding the advantages they offer and which mechanisms supply the best protection from herbi vores. Here, I test the prediction that the percent herbivory will decrease as pubescence and/or leaf toughness increases. MATERIALS AND METHODS Study Site Data were collected on plants growing in Lower Montane Rain Forest behind the EstaciÃ³n BiolÃ³gica on the Senderos Principal and DivisiÃ³n, and in the Monteverde Cloud Forest Reserve on the Sendero Nuboso, at Monteverde, Puntarenas Province, Costa Rica from March 16 to May 8, 2006.
Collection Seven species of Gesneriaceae were studied: Alloplectus te tragonus, Besleria princeps, Besleria solanoides, Besleria triflora, Columnea glabra, Columnea microcalyx, and Monopyle maxonii. The seven species are of diverse growth forms: three are herbs erect terrestrial herbaceous plants, three are shrubs woody p lants less than 5 m, and the remaining two, epiphytes Haber 2000. For each species, 20 individual plants were sampled and one leaf was taken from each. To minimize variation in leaf age, I collected the third leaf down from the tip of the branch. Perce nt Herbivory and Leaf Toughness For each leaf collected the percent herbivory and leaf toughness were measured on the day they were retrieved. The percent herbivory was determined using a leaf scanner. Two scans were performed for each leaf. First, the le af was scanned with area missing, assumed to be caused by herbivory. The second scan was done with a sheet of dark paper cut in the shape of the leaf and placed underneath to account for the total area. Percent herbivory was calculated by dividing the leaf with area missing by the total leaf area and subtracting this quotient from one. A penetrometer was used to measure leaf toughness. Coins of varying stacks were used to determine the relative toughness of the leaves. The number of coins needed to penetr ate the leaf was recorded and used to compare the toughness between the different groups species or leaf surface categories Figure 1. FIGURE 1. Penetrometer; apparatus used to measure leaf strength. a Leaf is placed in between top and bottom. b Top fits into bottom and holds leaf into place. c Blunt pin placed into hole on top of penetrometer. d Coins placed individually until pin penetrates through leaf providing a relative measurement of toughness.
Statistical Analyses Three leaf surface categories were identified and distinguished based on the amount or lack of pubescence present; glabrous 1 species, medium pubescence 4 species, and high pubescence 2 species. To compare the leaf surface and average percent herbivory, a 1 w ay ANOVA was run. To explore the relationship between herbivory and toughness, a Spearman rank correlation was performed to show across species variation and a series of simple regressions were used to investigate possible within species variation. In ord er to see if species vary in herbivory or toughness two 1 way ANOVAs were run, first comparing species and the average percent herbivory, and second comparing species and the average toughness. Lastly a 1 way ANOVA was run to compare leaf surface and avera ge leaf toughness. RESULTS Herbivory Percent missing from leaves in average was 0.0146, and ranged from 0.1334 to 0 n = 140. Leaf Surface Herbivory The leaf surface categories showed a significant difference in the average percent herbivory sugge sting that, with greater the amounts of pubescence, the average percent herbivory increases F value = 5.43, p value = 0.0054, df = 2 Figure 2. The glabrous leaf surface category showed significantly less herbivory than highly pubescent individuals Fis herÂ€s PLSD, p = 0.0016, and medium pubescence individuals showed significantly less herbivory than high pubescence individuals FisherÂ€s PLSD, p = 0.0355. The glabrous leaf surface category showed the least herbivory but it was not significantly lower th an the medium pubescence category for medium FisherÂ€s PLSD, p = 0.0614. FIGURE 2. The average percent differences in herbivory among leaf surface types of Gesneriads. As pubescence increases, percent herbivory increases. Glabrous 1 species, mediu m pubescence 4 species, high pubescence 2 species. Standard error bars represent Â± 1 standard error of the means. 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 Glabrous Medium Pubescence High Pubescence Leaf Surface Average Percent Herbivory n = 20 n = 80 n = 40
Relationship Between Herbivory and Toughness The Spearman rank correlation showed no significance across species when comparing average t oughness per species and average percent herbivory per species rho = 0.357, p value = 0.3817, n = 7 figure 3. There was also no significant effect of average percent herbivory on average leaf toughness when comparing within species relationships. The results of the simple regressions are shown in Table 1. FIGURE 3. The average toughness per species to average percent herbivory per species y = 0.0005x + 0.0216, R 2 = 0.1602. Across species relationship between herbivory and toughness showed no sig nificant results Spearman rank correlation, rho = .357, p value = .3817, n = 7. Circled, outlying point is B. triflora . TABLE 1. Simple regressions for each species show no significant relationship between the percent herbivory per leaf and the toughn ess of the leaf. Species R 2 p value n C. glabra 0.132 0.1149 20 B. solanoides 0.036 0.4210 20 B. triflora 0.003 0.8129 20 M. maxonii 0.014 0.6167 20 A. tetragonus 0.095 0.1858 20 B. princeps 0.100 0.1733 20 Species Variation There is signifi cant variation among the species when concerning average percent herbivory F value = 6.762, p value < 0.0001, df = 6 figure 4. There is significant variation among the species when concerning average leaf toughness F value = 158.204, p value < 0.0001, df = 2 figure 5. 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0 10 20 30 40 Average Toughness per Species Average Percent Herbivory per Species
FIGURE 4. Average percent herbivory observed among the seven different species of Gesneriads n = 20 per species. Leaf surface types is indicated for each species; glabrous solid, medium pubescence horizontal bars, and high pubescence diagonal lines. No herbivory observed for C. microcalyx . Letters indicate species with no significant difference. Standard error bars represent Â± 1 standard error of the means. 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 C. glabra B. solanoides B. triflora C. microcalyx M. maxonii A. tetragonus B. princeps Species Average Percent Herbivory a, d a, b, c e c, e d e b, e
FIGURE 5. Average leaf toughness observed among t he seven different species of Gesneriads n = 20 per species. Leaf surface types is indicated for each species; glabrous solid, medium pubescence horizontal bars, and high pubescence diagonal lines. Letters indicate species with no significant diffe rence. Standard error bars represent Â± 1 standard error of the means. Leaf Surface Toughness Leaf surface categories differed significantly in their leaf toughness suggesting the less pubescence, the tougher the leaf F value = 212.562, p value < 0.000 1, df = 2 figure 6. The glabrous leaf surface category showed significantly greater leaf strength than medium pubescence individuals FisherÂ€s PLSD, p < 0.0001 and high pubescence individuals FisherÂ€s PLSD, p < 0.0001. Medium pubescence individuals s howed significantly greater leaf strength than high pubescence individuals FisherÂ€s PLSD, p < 0.0001. 0 5 10 15 20 25 30 35 40 C. glabra B. solanoides B. triflora C. microcalyx M. maxonii A. tetragonus B. princeps Species Average Leaf Toughness a a a, b b c c d
FIGURE 6. The average leaf toughness among different leaf surface types of Gesneriads. Glabrous 1 species, medium pubescence 4 species, high pubescence 2 species. As leaf surface becomes more hairy, the leaf toughness decreases. Standard error bars represent Â± 1 standard error of the means. DISCUSSION Plant physical defenses represent energy allocation that could have been re routed to exp enditures on other life history attributes such as reproduction Coley 1983. The desired pay off for investing energy into antiherbivore mechanisms is an increase in fitness. The purpose of this study was to determine if physical defenses hairs and tough ness employed by Gesneriaceae effectively deter herbivores. The percent herbivory was predicted to decrease with increasing pubescence and toughness. The questions posed are: Do leaf surface and/or leaf toughness affect herbivory? Is there a relationship between leaf surface and toughness? And, do species vary in herbivory and/or toughness? Contrary to what was predicted, as pubescence increases so does the percent of herbivory. This is probably not due to a direct positive effect of hairs on herbivores, rather, it may be due to the negative correlation with more effective defenses Coley 1983. In some cases it seems that hairs are successful in deterring herbivores. For instance, Gilbert 1971 showed that the hooked hairs of Passiflora adenopoda prohib ited the movement of and even wounded heliconiine butterfly caterpillars attempting to feed upon the vegetative tissues. However, for members of the family Gesneriaceae studied here, pubescence does not appear to be effective against herbivory. Coley 1983 suggests that pubescence appears to be the best single, readily observed character for identifying poorly defended leaves or plants. It is possible that Gesneriads employ hairs to compensate for the lack of more effective defenses. Perhaps they are attem pting to skimp on defense investments to allocate energy elsewhere such as growth and reproduction. Yet another possibility is that pubescence has different function for gesneriads. The main effect of hairs was once considered to be reducing transpiration by thickening the leaf boundary layer. In other cases pubescence undoubtedly increases leaf reflectivity, consequently lowering leaf temperature and rates or water loss Fitter and Hay 1987; Raven et al. 1999. Most, if not all, of the leaves sampled were from plants directly on the trail where light levels are slightly higher. Based on their locations it could be beneficial 0 5 10 15 20 25 30 35 40 Glabrous Medium Pubescence High Pubescence Leaf Surface Average Leaf Toughness n = 20 n = 40 n = 80
for these Gesneriads to use pubescence to increase reflectivity to have better control of their temperature and thus transpiration ra tes. A paradox arises with the relationship between percent herbivory and toughness. Leaf surface is related to herbivory and toughness is related to leaf surface, thus it seems logical that herbivory and toughness would also be related. As pubescence inc reases, herbivory increases, and toughness decreases, however, when compared exclusively to each other percent herbivory and leaf toughness show no significant relationship. Neither across species or within species analyses displays significant results. Fr om the herbivore perspective, a relationship would be reasonable to suspect because as the leaf is tougher, for many it would be more difficult to feed upon. Similarly, the plant is seemingly allocating resources into toughening the leaf, thus herbivory wo uld expectantly be lower. A case specific reason for the lack of a significant relationship is the small sample size. The outlier highlighted in figure three, B. princeps, drastically affects the outcome by falling outside the overall pattern. Besleria pri nceps posed a unique issue compared to the other species of Gesneriads sampled. Based on observations, many times herbivory increased from the second leaf to the third. It seems as though other factors may be combined with leaf toughness that influences the frequency of herbivory. For B. princeps it may be possible that leaf age is affecting the rate of herbivory. In general, perhaps the less tough leaves have little nutritional value and thus are not highly fed upon because it is not energetically advantage ous for the herbivore. Another possibility is the presence of secondary compounds, many of which are toxic to fungi, insects, or other enemies Fitter and Hay 1987. Perhaps secondary compounds are present in the leaves that are less tough resulting in the deterrence of herbivores. Next, results here suggest that less pubescence, the tougher the leaf. This could represent a trade off of different defense mechanisms. Plants must create a balance between the energy spent on defense against herbivores and the energy allocated to other functions such as growth or reproduction. They need to maintain the defenses necessary to survive in the presence of pathogens and herbivores; however, they must also grow fast enough to compete Herms and Mattson 1992. Results indicated significant differences between species and percent herbivory, and between species and leaf toughness. Coley 1983 highlights the differences among species and herbivory rates in a tropical forest where insects remove from 0.0003 to 0.8 percent of the leaf area per day, depending on the tree species. The range of effects on different species many times reflects simple palatability differences Coley 1985. Variation in the amount of herbivory different species experience could also be due to the array of defenses that each species employs. One obvious example is the difference in leaf toughness shown by the current study. The species included in this study encompass a variety of growth forms that could contribute to the differences among herbivory and leaf toughness. Columnea glabra stood out from the other species in toughness thus illustrating how different growth forms may have an effect on how defense mechanisms are employed. An epiphyte may utilize different characteristics than a terrestrial shrub due to the type of habitat it occupies. As you move up into the canopy, wind levels increase along with the risk of desiccation, thus more succulent leaves may be favored. Overall, Gesneriaceae may not present an accurate picture of the effective ness of these physical defenses. The herbivory rates are rather low which could be due to a number of reasons. These defenses could be effectively performing their duty and
deterring herbivores, or maybe Gesneriads simply are not tasty or nutritious enough thus herbivores could prefer other sources. The results suggest that pubescence is not an effective mode of physical defense for the family Gesneriaceae. Increasing pubescence did not deter herbivores, thus it could point toward different functions that are more primary. Leaf toughness was not directly related to herbivory, although with a greater sample size a clearer trend may appear. In addition to increasing sample size, increasing the number of glabrous species would be beneficial to see if trends f ollow the same pattern. Determining the presence of secondary compounds or nutritive values of the leaves would also help to see which mechanisms are the most effective defenses against herbivory. ACKNOWLEDGEMENTS I would like to thank my advisor Kare n Masters for her calculated and caring guidance throughout this project. Thank you to all of the professors; Alan Masters, Karen Masters, Javier MÃ©ndez, and Carmen Rojas, for giving me wonderful models of teachers that combine knowledge, experience, and p ersonality, and constantly support their students on many levels. Thanks to our rad TAs, Maria Jost and Oliver Hyman, for incessantly helping us and providing sweet entertainment along the way. A special thanks to La EstaciÃ³n BiolÃ³gica for being home inclu ding the most awesome backyard ever. Thanks Rachel for keeping me on the ball and almost sane. T. Trio forever. Thank you CIEE Spring 2006. LITERATURE CITED Coley, P. D. 1983. Herbivory and defensive characteristics of tree species in a lowland tropica l forest. Ecology Monographs 53 : 209 229. Coley, P. D., Bryant, J. P., and Chapin III, S. 1985. Resource availability and plant antiherbivore defense. Science 230 : 895 899. Fitter, A. H. and R. K. M. Hay. 1987. Environmental physiology of plants. Academi c Press, San Diego. Gentry, A. H. 1993. A field guide to the families and genera of woody plants of Northwest South America Colombia, Ecuador, Peru with supplementary notes on herbaceous taxa. The University of Chicago Press, Chicago and London. Gilber t, L. E. 1971. Butterfly: plant co evolution: has Passiflora adenopoda won the selectional race with Heliconid butterflies? Science 172 : 585 586 Herms, D. A., and Mattson, W. J. 1992. The dilemma of plants: to grow or to defend. The Quarterly Review of B iology 67 : 283 310. Herrera, C. M., and O. Pellmyr. 2002. Plant animal interactions; an evolutionary approach. Blackwell Science Ltd, Oxford. Nadkarni, N. M., and Wheelwright, N. T. 2000. Monteverde; ecology and conservation of a tropical cloud forest. O xford University Press, New York and Oxford. Osborne, P. L. 2000. Tropical ecosystems and ecological concepts. Cambridge University Press, Cambridge, United Kingdom. Raven, P. H., Evert, R. F., and Eichhorn, S. E. 1999. The biology of plants; sixth editi on. W. H. Freeman and Company/Worth Publishers, New York. Simms, E. L., and Rausher, M. D. 1987. Costs and benefits of plant resistance to herbivory. The American Naturalist 130 : 570 581. Struass, S. Y., and Agrawal, A. A. 1999 The ecology and evolution of plant tolerance to herbivory. TREE 14 : 179 185. Zuchowski, W. 2005. A guide to tropical plants of Costa Rica. Zona Tropical Publication , Miami, Florida.